1. Field of the Invention
This application relates to hearing aids. The invention, more specifically, relates to hearing aids having means for reproducing sounds at frequencies otherwise beyond the perceptive limits of a hearing-impaired user. The invention further relates to a method of processing signals in a hearing aid.
Individuals with a degraded auditory perception are in many ways inconvenienced or disadvantaged in life. Provided a residue of perception exists they may, however, benefit from using a hearing aid, i.e. an electronic device adapted for amplifying the ambient sound suitably to offset the hearing deficiency. Usually, the hearing deficiency will be established at various frequencies and the hearing aid will be tailored to provide selective amplification as a function of frequency in order to compensate the hearing loss according to those frequencies.
A hearing aid is defined as a small, battery-powered device, comprising a microphone, an audio processor and an acoustic output transducer, configured to be worn in or behind the ear by a hearing-impaired person. By fitting the hearing aid according to a prescription calculated from a measurement of a hearing loss of the user, the hearing aid may amplify certain frequency bands in order to compensate the hearing loss in those frequency bands. In order to provide an accurate and flexible amplification, most modern hearing aids are of the digital variety. Digital hearing aids incorporate a digital signal processor for processing audio signals from the microphone into electrical signals suitable for driving the acoustic output transducer according to the prescription.
However, there are individuals with a very profound hearing loss at high frequencies who do not gain any improvement in speech perception by amplification of those frequencies. Hearing ability could be close to normal at low frequencies while decreasing dramatically at high frequencies. These steeply sloping hearing losses are also referred to as ski-slope hearing losses due to the very characteristic curve for representing such a loss in an audiogram. Steeply sloping hearing losses are of the sensorineural type, which are the result of damaged hair cells in the cochlea.
People without acoustic perception in the higher frequencies (typically from between 2-8 kHz and above) have difficulties regarding not only their perception of speech, but also their perception of other useful sounds occurring in a modern society. Sounds of this kind may be alarm sounds, doorbells, ringing telephones, or birds singing, or they may be certain traffic sounds, or changes in sounds from machinery demanding immediate attention. For instance, unusual squeaking sounds from a bearing in a washing machine may attract the attention of a person with normal hearing so that measures may be taken in order to get the bearing fixed or replaced before a breakdown or a hazardous condition occurs. A person with a profound high frequency hearing loss, beyond the capabilities of the latest state-of-the-art hearing aid, may let this sound go on completely unnoticed because the main frequency components in the sound lie outside the person's effective auditory range even when aided.
High frequency information may, however, be conveyed in an alternative way to a person incapable of perceiving acoustic energy in the upper frequencies. This alternative method involves transposing a selected range or band of frequencies from a part of the frequency spectrum imperceptible to a person having a hearing loss to another part of the frequency spectrum where the same person still has at least some hearing ability remaining.
2. The Prior Art
WO-A1-2007/000161 provides a hearing aid having means for reproducing frequencies originating outside the perceivable audio frequency range of a hearing aid user. An imperceptible frequency range, denoted the source band, is selected and, after suitable band-limitation, transposed in frequency to the perceivable audio frequency range, denoted the target band, of the hearing aid user, and mixed with an untransposed part of the signal there. For selecting the frequency shift, the device is adapted for detecting and tracking a dominant frequency in the source band and a dominant frequency in the target band and using these frequencies to determine with greater accuracy how far the source band should be transposed in order to make the transposed dominant frequency in the source band coincide with the dominant frequency in the target band. This tracking is preferably carried out by an adaptable notch filter, where the adaptation is capable of moving the center frequency of the notch filter towards a dominant frequency in the source band in such a way that the output from the notch filter is minimized. This will be the case when the center frequency of the notch filter coincides with the dominating frequency.
The target frequency band usually comprises lower frequencies than the source frequency band, although this needs not necessarily be the case. The dominant frequency in the source band and the dominant frequency in the target band are both presumed to be harmonics of the same fundamental. The transposition is based on the assumption that a dominant frequency in the source band and a dominant frequency in the target band always have a mutual, fixed, integer relationship, e.g. if the dominant frequency in the source band is an octave above a corresponding, dominant frequency in the target band, that fixed integer relationship is 2. Thus, if the source band is transposed an appropriate distance down in frequency, the transposed, dominant source frequency will coincide with a corresponding frequency in the target band at a frequency one octave below. The inventor has discovered that, in some cases, this assumption may be incomplete. This will be described in further detail in the following.
Consider a naturally occurring sound consisting of a fundamental frequency and a number of harmonic frequencies. This sound may e.g. originate from a musical instrument or some natural phenomenon like e.g. birdsong or the voice of someone speaking. In a first case, the dominant frequency in the source band may be an even harmonic of the fundamental frequency, i.e. the frequency of the harmonic may be obtained by multiplying the frequency of the fundamental by an even number. In a second case, the dominant harmonic frequency may be an odd harmonic of the fundamental frequency, i.e. the frequency of the harmonic may be obtained by multiplying the frequency of the fundamental with an odd number.
If the dominant harmonic frequency in the source frequency band is an even harmonic of a fundamental frequency in the target band, the transposer algorithm of the above-mentioned prior art is always capable of transposing the source frequency band in such a way that the transposed dominant harmonic frequency coincides with another harmonic frequency in the target frequency band. If, however, the dominant harmonic frequency in the source frequency band is an odd harmonic of the fundamental frequency, the dominant source frequency no longer shares a mutual, fixed, integer relationship with any frequency present in the target band, and the transposed source frequency band will therefore not coincide with a corresponding, harmonic frequency in the target frequency band.
The resulting sound of the combined target band and the transposed source band may thus appear confusing and unpleasant to the listener, as an identifiable relationship between the sound of the target band and the transposed source band is no longer present in the combined sound.
Another inherent problem with the transposer algorithm of the prior art is that it does not take the presence of speech into account when transposing the signal. If voiced-speech signals are transposed according to the prior art algorithm, formants present in the speech signals will be transposed along with the rest of the signal. This may lead to a severe loss of intelligibility, since formant frequencies are an important key feature to the speech comprehension process in the human brain. Unvoiced-speech signals, however, like plosives or fricatives, may actually benefit from transposition, especially in cases where the frequencies of the unvoiced-speech signals fall outside the perceivable frequency range of the hearing-impaired user.